Colour of exhaust smoke

How to tell the condition of the engine by the colour of exhaust smoke.

Exhaust smoke should be colourless but it sometimes isn’t so; in certain conditions, it may be black, white, or even blue. While these changes are bound to attract your attention, they don’t necessarily mean something wrong is happening; still, we always recommend taking the car to the station to make sure it is examined by a specialist. This is just as important when you are buying a second-hand car as it is when you notice it in the car you already have; as an ordinary driver you have no chance to recognise the warning signs a professional won’t miss. Tell the mechanic why you have come and let them check the car; if the change of smoke colour suggests that serious repairs are coming, the sooner you make them, the cheaper they will be. Our guide is not meant to replace professional advice but it will help you understand what may be happening inside your car.

Black smoke

The fact that engine smoke has become black does not mean a serious breakdown; for example, some diesel vehicles will always produce black smoke when the engine is heavily loaded. If you can see black smoke often, the injection system may need checking as this is typically caused by low quality of fuel or excessive wear of the injectors. If you’re lucky, you may only need to change the intercooler pipes; this would increase the engine power and make its work quieter.

When should you be worried, then? Black smoke is always suspicious if your diesel car is relatively new; old diesels are known for the black clouds of smoke they produce and while unpleasant, they don’t need to suggest any fault. When it comes to petrol engines, black smoke often suggests faulty, amateur tuning.

Grey smoke

Exhaust smoke turns grey when the proportions of fuel and smoke change in the engine. One of the more common reasons is a faulty engine sensor, e.g. the temperature sensor. When the computer cannot tell the exact temperature if the engine, it will provide more fuel, thus making the mixture too rich. This will, of course, increase fuel consumption as well. Grey smoke hardly ever appears in the exhaust pipe on its own so if you can see it on a regular basis, make sure you visit a station as quickly as you can.

Blue smoke

When exhaust smoke turns blue, too much of engine oil is burnt together with the fuel. You can check oil consumption yourself, of course; do it on a regular basis and if it increases, you will surely notice the difference. Blue smoke can appear when you have a faulty turbo or valve seals – these are the more optimistic scenarios. If you are unlucky, the smoke can suggest faulty piston rings or sleeves; this means you need to prepare yourself for serious (and costly) engine repairs.

White smoke

Exhaust smoke often turns white in winter; this is caused by vaporisation of the water collected in the exhaust system, and vapour is always visible when the temperature drops below zero. The smoke should get back to normal as you are driving, though. If it doesn’t, you may have a problem with the cylinder head or the seal below it. If cooling liquid gets through to the exhaustion system, there will be water in there, so you will definitely see vapour. You would also notice constant or temporary power decrease.